Process for producing powder coating composition and powder coating composition obtained by the production process

ABSTRACT

A production process characterized by fluidizing resin particles as a base powder and spraying the fluidized particles with a slurry containing a pigment dispersed therein to coat the surface of the resin particles with the pigment.

TECHNICAL FIELD

The present invention relates to a process for producing a powdercoating composition in which the surface of a base powder of syntheticresin is coated with a pigment prepared in liquid form. The presetinvention also relates to a powder coating composition produced by sucha process.

BACKGROUND ART

Conventionally widely used powder coating compositions include those inwhich the surface of a base powder of synthetic resin is coated with apigment, in which case the product is called a colorant powder coatingcomposition, or with a metal foil powder, in which case the product iscalled a metallic powder coating composition. Such colorant and metallicpowder coating compositions are generally produced by dry blending. Dryblending is simple blending of the powder of the resin used as a basepowder with the powder of a pigment or a metal foil powder under dryconditions; specifically, in a mixer or the like, a resin powder ismixed with predetermined additives, pigments, functional resins, and thelike.

An example of such a process is disclosed in Japanese Patent ApplicationLaid-open No. H10-279695. According to the technique disclosed there, apigment in the form of colorant particles is melted by the use of akneader, and is then crushed in a liquid by the use of a roll mill, abead mill, or the like to produce a liquid having the pigment dispersedtherein. This liquid is then added to a base powder of a powder coatingcomposition, and these are then mixed together by dry blending by theuse of a mixer.

To cope with the recent trend of demands for such colorant and metallicpowder coating compositions, it is nowadays necessary to produce them ina wide variety, that is, with different colors, patterns, glosses, andother design features, but in small quantities (in batches of severalkilograms to ten and several kilograms). For this purpose, however, theconventional process described above is unsuitable because of thefollowing disadvantages. When a colorant or metallic powder coatingcomposition is produced by the conventional process, a base resin powderand a pigment or a metal foil powder need to be mixed togetherbeforehand. Thus, for example, when the color of a colorant powdercoating composition being produced is changed, cleaning is needed in alarge part of the production line, such cleaning disadvantageouslyrequiring large manpower and high cost. Moreover, it is generallybelieved that the minimum batch of a powder coating composition thatjustifies such requirements is about one metric ton, and this makes itimpracticable to produce, by the conventional process, powder coatingcompositions in small units of several kilograms to ten and severalkilograms. Thus, even when a wide variety of powder coating compositionsare needed in small quantities, they need to be purchased in quantitieslarger than necessary. That is, with the conventional process,disadvantageously, it is impracticable to cope with wide-varietysmall-quantity production.

Moreover, in the conventional process described above, for the purposeof dispersing a pigment as evenly as possible, it is necessary toperform mixing using a mixer for a long time. When a thermally curablepowder coating composition is produced, such long-time mixing raises thetemperature inside the mixer, with the result that, disadvantageously, acurable agent mixed together causes part of the resin to gelate. Thismakes it difficult to produce such a powder coating composition. Thisdisadvantage is particularly notable in the production of a thermallycurable powder coating composition that can be baked at low temperature.

To overcome these disadvantages, and to cope with wide-varietysmall-quantity production, studies have been done in search of a processby which first a base powder coating composition is produced and thendesired design features are added to it. An example of such process isdisclosed in Japanese Patent Application Laid-open No. 2001-205186.According to the technique disclosed there, in a colorant powder coatingcomposition produced by dry blending, a base powder of the powdercoating composition and a colorant powder are prepared separately, andthey are, immediately before application, mixed evenly by the use of amixer so that the coating composition is obtained in the desiredquantity and color. This makes wide-variety small-quantity productionpossible.

The dry blending described above, however, has the followingdisadvantages. The colorant powder that attaches to the surface of thebase powder is highly flocculative, and moreover, in particular when thecolorant powder is a fine one with a particle size of severalmicrometers or less, it is also poorly dispersive. Thus, it ispractically impossible to disperse such a fine colorant powder so thatit coats the surface of the base powder. Inconveniently, this results inpoor adhesion (low adhesion strength) between the resin powder and thepigment.

Also with a metal foil powder, the dry blending described above has thefollowing disadvantages. The adhesion between the resin powder and themetal foil powder is poor, and moreover, during application, the resinpowder and the metal foil powder tend to separate at the tip of anapplicator gun. This results in poor workability. Moreover, the metalfoil powder that has separated at the tip of the applicator gun attachesand deposits there. From time to time, the deposit leaves the tip of theapplicator and attaches to the coating surface, causing a coating defectcalled spit. This spoils the design of the article coated.

Furthermore, since, as described above, the adhesion between a resinpowder and a pigment or a metal foil powder is poor, the two tend toseparate during application. Thus, the powder coating compositionretrieved for reuse after application usually contains the separatedpigment or metal foil powder. This makes the reuse of the retrievedpowder difficult.

To overcome these disadvantages, according to the process disclosed inJapanese Patent Application Laid-open No. 2002-338895, in an upwardcurrent of a gas containing oxygen, while a base powder is fluidized andcirculated in the up and down directions, the base powder is irradiatedwith ultraviolet ray so that active points are formed on the surface ofthe base powder, and, with the base powder in this state, a pigment inthe form of powder or fine flakes is brought into contact.

According to another conventionally disclosed process, in a fluidizedbed, a resin powder as a base powder and a pigment or a metal foilpowder are dispersed and mixed together, and moreover, for the purposeof binding their particles together, during the mixing, a binder in theform of spray is supplied into the fluidized bed (for example, seeJapanese Patent Application Laid-open No. 2004-2633).

As compared with dry blending, the above-described process involving theirradiation of ultraviolet rays does help improve the adhesion betweenthe resin powder and the pigment, but has the following disadvantages.The pigment in a gas current is not very dispersive, and therefore, withits own action in a gas current alone, it is difficult to coat thesurface of the resin powder satisfactorily with the dispersed pigmenteven when it is supplied in the form of highly flocculative powder orfine flakes. Moreover, the pigment, when it coats the surface of theresin powder, is in a flocculated state, and this makes it difficult toobtain an even color in the appearance of a coating finished with acolorant powder coating composition produced by the above-describedprocess involving the irradiation of ultraviolet rays. To improve this,when the colorant powder coating composition is produced, more of theabove-mentioned pigment needs to be added to it. These disadvantages areencountered also when, instead of a pigment, a fine metal foil powder isused.

As compared with dry blending, the above-described process involving thesupply of a binder in the form of spray does help improve the adhesionbetween the resin powder and the pigment or the metal foil powder, buthas the following disadvantages. In the appearance of the finishedcoating, it is sometimes difficult to obtain an even color and gloss. Inparticular with a metallic powder coating composition containing a metalfoil powder, disadvantageously, a satisfactorily metallic luster is notobtained in the appearance of the finished coating.

In view of the conventionally encountered disadvantages discussed above,it is an object of the present invention to provide a process forproducing a powder coating composition whereby the surface of a basepowder is coated with a pigment made highly dispersive so that increasedadhesion is obtained between the resin powder and the pigment, theprocess being capable of coping with wide-variety small-quantityproduction, and to provide a powder coating composition produced by sucha process.

DISCLOSURE OF THE INVENTION

To achieve the above object, according to the present invention, aprocess for producing a powder coating composition involves fluidizing aresin powder as a base powder and supplying a liquid in a form of slurryhaving a pigment dispersed therein in such a way that a spray of thepigment hits the fluidized resin powder so that the surface of the resinpowder is coated with the pigment. Here, mixing a binder in the liquidin the form of slurry helps obtain increased coating stability. Thepigment may be a common pigment (a pigment in the form of fine powder orfine flakes), or a metal foil powder, or a mixture of those.

It is preferable that the resin powder have a mean particle size of 5 to50 μm.

When a common pigment is used, it is preferable that it have a meanparticle size of 0.001 to 50 μm, and it is preferable that the resinpowder and the pigment be mixed together in a ratio of, on a weightpercentage basis, 100 of the resin powder to 0.5 to 40 of the pigment.

When a metal foil powder is used, it is preferable that it have a meanparticle size of 1 to 50 μm, and it is preferable that the resin powderand the metal foil powder be mixed together in a ratio of, on a weightpercentage basis, 100 of the resin powder to 0.5 to 15 of the metal foilpowder.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical sectional view schematically showing, as a firstembodiment of the present invention, a powder treatment apparatus usedin a process for producing a colorant or metallic powder coatingcomposition; and

FIG. 2 is a vertical sectional view schematically showing, as a secondembodiment of the present invention, a powder treatment apparatus usedin a process for producing a colorant or metallic powder coatingcomposition.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. FIG. 1 is a vertical sectionalview schematically showing, as a first embodiment of the presentinvention, a powder treatment apparatus used in a process for producinga colorant or metallic powder coating composition.

In FIG. 1, reference numeral 2 indicates a treatment apparatus main unithaving a treatment chamber 1, and reference numeral 3 represents apowder material supply port provided on a wall 2 a of the treatmentapparatus main unit 2. In this embodiment, an upward current of air ispassed through the interior of the treatment chamber 1 so that afluidized bed is produced in a treatment area A located in a lower partof the interior of the treatment chamber 1. On the other hand, a liquid(a liquid in the form of suspended slurry) having dispersed therein apigment (or a metal foil powder) in the form of highly flocculative finepowder or fine flakes is prepared, and a spray of this liquid in theform of suspended slurry is supplied into the fluidized bed; that is, itis supplied through the powder material supply port 3 into the treatmentchamber 1. This causes the pigment (or metal foil powder) to coat thesurface of the powder material (resin powder) in a fluidized state. Thatis, while a resin powder as a base powder is fluidized, a liquid in theform of slurry having a pigment (or metal foil powder) in the form offine powder or fine flakes dispersed therein is supplied in such a waythat a spray of the pigment hits the fluidized resin powder. Thispermits the surface of the resin powder to be coated with the pigment(or metal foil powder). Here, fluidization is achieved by the use offluidizing air alone.

For the production of a colorant powder coating composition in which thesurface of a resin powder as a base powder is coated with a pigment,preferred examples of the material of the resin powder include:synthetic resins such as polyester-urethane curable resin,epoxy-polyester curable resin, epoxy resin, acrylic resin,acrylic-polyester resin, fluororesin, acrylic-urethane curable resin,acrylic-melamine curable resin, and polyester-melamine curable resin.One of these materials is used singly or two or more of them are used incombination to prepare the base powder, with appropriate quantities ofmodifier and plasticizer added thereto as necessary. Examples ofmodifiers include: novolac resin, phenoxy resin, butyral resin, ketoneresin, polyester resin, and rosin. Example of plasticizers include:epoxidized oil and dioctyl phthalate.

Preferred examples of the material of the pigment includes: colorantpigments such as titanium dioxide, iron black, iron red, iron oxides,zinc dust, antimony white, carbon black, pigment yellow, mapico yellow,red lead, cadmium yellow, zinc sulfide, lithopone barium sulfate, leadsulfate, barium carbonate, calcium carbonate, white lead, alumina white,phthalocyanine pigments, quinacridone pigments, azo pigments,isoindolinone pigments, flavanthrone pigments, anthraquinone pigments,anthrapyridine pigments, pyranthrone pigments, dioxazine pigments,perylene pigments, perinone pigments, and various baked pigments; andextender pigments such as silica, talc, barium sulfate, calciumcarbonate, and glass flakes.

The process of this embodiment for producing a colorant powder coatingcomposition is particularly suitable for the production of colorantpowder coating compositions with a mean particle size of 5 to 50 μm.Accordingly, as the resin powder supplied, one with a mean particle sizeof 5 to 50 μm is used, and, as the pigment, one with a mean particlesize of 0.001 to 50 μm is used. The resin powder and the pigment aremixed together in a ratio of, on a weight percentage basis, 100 of theresin powder to 0.5 to 40 of the pigment. The rate at which the air forfluidizing the resin powder is supplied is, on a void tower speed basis,0.15 m/s to 1.2 m/s (converted in zero-degree, one-atmosphere terms).These conditions are common to this and the later-described secondembodiments.

On the other hand, for the production of a metallic powder coatingcomposition in which the surface of a resin powder is coated with ametal foil powder, preferred examples of the material of the resinpowder include epoxy, polyester, and acrylic, and preferred examples ofthe material of the metal foil powder include aluminum. As the resinpowder, one with a mean particle size of 5 to 50 μm is used, and, as themetal foil powder, one with a mean particle size of 1 to 50 μm is used.The resin powder and the metal foil powder are mixed together in a ratioof, on a weight percentage basis, 100 of the resin powder to 0.5 to 15of the metal foil powder.

As shown in FIG. 1, in a lower part of the treatment apparatus main unit2, there are provided: an air supply port 10 via which air is suppliedinto the treatment chamber 1; a blower 11; and a heater 12. Through anair blow outlet 5 provided in a lower part of the treatment chamber 1,air heated by the heater 12 is blown upward into the treatment chamber1. This air fluidizes the resin powder (base powder) while keeping it ina fluidized bed and in a predetermined range of temperature. Here, thepredetermined range of temperature is equal to or lower than the meltingpoint of the resin. The resin powder is then kept in this state for apredetermined time, for example five minutes or more.

The air blow outlet 5 is a air blow member 5 a that has a large numberof openings 5 b formed therethrough and that is fitted in a lower partof the treatment chamber 1 so as to close it there. Preferably, thepressure of the air introduced through the air blow outlet 5 is so setthat individual particles are fluidized to circulate inside thetreatment chamber 1.

The blow of air through the air blow outlet 5 is largely in the upwarddirection. Specifically, the direction may be such that air is blownvertically upward, or, by the use of protruding slit windows, obliquelyupward so that a swirling current of air is formed inside the treatmentchamber 1. Examples of the air blow member 5 a of the type that permitsair to be blown upward include a sintered metal mesh and a punchedplate; examples of the type that produces a swirling current of airinclude a screen having protruding slit windows, which is a type ofpunched plate. Here, the aperture ratio is set roughly in the range from3% to 12%; in this particular embodiment, it is set at about 5%regardless of the type of the air blow member 5 a. Producing a swirlingcurrent of air as described above permits particles to be swirled upwardmore effectively, and thus helps make the temperature distributioninside the fluidized bed more even. Although not illustrated, separatelyfrom the air introduced through the air blow outlet 5, pressurized airmay be supplied into the fluidized bed intermittently.

The state of mix of the particles inside the fluidized bed can beimproved by the use of a so-called agitator that exerts a physicallyagitating effect, or a rotary disk that exerts a rolling effect.However, the use of a mechanism like these often causes the crushing ofthe metal foil powder, and thus adversely affects the color andweather-resistance of the product. In such a case, the formation of aswirling current of air or the supply of pressurized air as describedabove is effective. During powder treatment, the bonding of the resinpowder and the metal foil powder may be promoted through the supply of aspray of a binder such as a coupler. To prevent dust explosion, ionizedair may be supplied.

Next, a description will be given of the preparation of a liquid in theform of suspended slurry (hereinafter referred to simply as slurry)containing a pigment (or metal foil powder) in the form of highlyflocculative fine powder or fine flakes. As shown in FIG. 1, in thisembodiment, slurry is prepared by the use of a wet ball mill 19. Thiswet ball mill 19 is composed of an upright cylindrical vessel that hasan agitating roller 21 arranged inside it and of which the interior isfilled with balls 17 (for example, balls of zirconia) with innerdiameters of 1 to 5 mm. When the agitating roller 21 is rotated, theballs 17 agitate the interior of the wet ball mill 19 so that thepigment or metal foil powder supplied into the wet ball mill 19 ismilled into well dispersed fine powder or fine flakes.

More specifically, as shown in FIG. 1, the pigment (or metal foilpowder) and water are, along with a binder used for the purpose ofbinding particles together, supplied into the wet ball mill 19. Sincethe interior of the wet ball mill 19 is filled with the balls 17, whenthe wet ball mill 19 is operated at a predetermined rotation rate for apredetermined time, slurry 13 is prepared. At the outlet of the wet ballmill 19 is provided a slurry tank 18 so that the prepared slurry 13 isstored in the slurry tank 18. Instead of using the wet ball mill 19, itis also possible to prepare slurry 13 by supplying the pigment, water,and a binder used for the purpose of binding particles together into abeaker and then agitating them by the use of a stirrer or a homogenizer.In particular when a metal foil powder in the form of flakes is formedinto slurry, with a view to preventing the breakage of the metal foil,it is preferable to adopt an agitating method that uses a comparativelymild agitating force as achieved by the use of a stirrer or the like.

Moreover, as shown in FIG. 1, under the treatment area A, spray nozzles4 are provided to point upward. By the action of a pump 14, the slurry13 is supplied to the spray nozzles 4 as indicated by thick broken linearrows in the figure, and spray air is also supplied to the spraynozzles 4 as indicated by arrows S in the figure. In this way, a sprayof the slurry 13 is supplied into the fluidized bed in the treatmentarea A. Although not illustrated, spray nozzles 4 may be provided alsoabove the treatment area A to point downward.

Here, used as the binder is, among others:

1. one, such as a solvent, that enhances the tucking property of thepowder coating composition resin itself; or

2. one that itself is an additive (coupler) for adding an adhesiveproperty and that is dissolved, diluted, or dispersed in water or in avolatile organic solvent.

That is, the binder mixed in the slurry 13 is used for the purpose ofincreasing the bonding power between the resin powder and the pigment(or metal foil powder). It should be noted that the binder needs to beselected to suit the resin. In this embodiment, a binder classified to2. above is preferably used.

Examples of couplers classified to 2. above include: those based onacrylic, acrylic acid, urethane resin, polyethylene resin, epoxy resin,polyethylene glycol, polyvinyl chloride, polyester, polypropylene,polybutadiene, polystyrene, phenol resin, metacrylic acid, terephthalicacid, acrylonitril, AS resin, ABS resin, vinyl chloride resin,fluororesin, polyvinyl alcohol, maleic acid resin, methacrylic acidresin, polyacetal, polycarbonate, alkyd resin, polyethyleneterephthalate (PET) resin, polyamide resin, urea resin, melamine resin,phenol resin, silicone resin, terpene resin, vinyl, vinyl chloride,nylon, polyvinyl alcohol, cellulose, saccharides, and natural rubber.Any of these materials is used in the form dissolved or diluted in wateror an organic solvent, or in the form of dispersed particles as by beingcrushed into particles and dispersed in water, an organic solvent, orthe like so as to be prepared as slurry or suspended liquid, or by beingformed into finer particles and dispersed so as to be prepared as acolloid, latex, or elastomer. When a binder in the form of dispersedparticles is used, the smaller the particle size, the better.Specifically, it is preferable that the particle size be 0.1 μm to 0.3μm. It is preferable that the binder liquid concentration (theconcentration by weight of the coupler relative to the whole binderliquid) be in the range from 0.1 to 10% by weight. A diluted ordissolved additive may be supplied into the binder liquid (in thisembodiment, the slurry 13).

Preferred among the various kinds of binder named above are those basedon urethane resin, and it is particularly preferable to use water-basedurethane resin as a coupler. One example of such a binder usingwater-based urethane resin is water-based urethane resin having awell-known urethane elastomer dispersed in water (manufactured under theproduct name “Superflex” by Dai-ichi Kogyo Seiyaku Co., Ltd.).

When a spray of the slurry 13 is supplied to hit the resin powder in thefluidized bed, for example, a spray of a water-based binder using theabove-mentioned water-based urethane resin as a coupler is supplied intothe fluidized bed. The water-based binder is not hazardous as is oneusing a organic solvent. This eliminates the need to worry about theresidual organic solvent in the product. Thus, it is possible to achievesafer, more efficient production, and to prevent environmentalpollution.

When a binder classified to 2. above is used, it is preferable that atleast 10 ml of the binder liquid (that is, the slurry 13) in thedissolved (or diluted or dispersed) form be supplied for every 1 kg massof the powder supplied into the fluidized bed.

The liquid (water, a volatile organic solvent, or the like; in thisembodiment, water) in which the coupler is dispersed, dissolved, ordiluted is selected on the basis of the flocculating power thereof. Asan indicator of the flocculating power, it is possible to use thesolubility factor (also called solubility coefficient; hereinafterreferred to as the “SP value”), which equals the square root of theflocculating energy density of molecules. When the SP value of theliquid and the SP value of the powder coating composition resin areclose together, the liquid comes close to the surface of the powdercoating composition resin, and this makes it easier for the liquid toseep into the powder coating composition resin. As a result, after theliquid along with the coupler is supplied as the binder to the powdercoating composition resin, disadvantageously, it takes an extremely longtime to perform a drying process for removing the liquid from the powdercoating composition resin. To avoid this, it is preferable that the SPvalue of the liquid in which the coupler is dispersed, dissolved, ordiluted be ±1 or more apart from the SP value of the powder coatingcomposition resin.

In this embodiment, first, at a predetermined temperature, the resinpowder is fluidized inside the treatment chamber 1, and then, as thetemperature of the powder is raised, a spray of the slurry 13 issupplied at a predetermined supply rate into the fluidized bed in thetreatment area A. Thereafter, drying is performed in the treatment areaA. Through this procedure, the surface of the resin powder is coatedwith the pigment or metal foil powder dispersed in the slurry 13,achieving the production of a colorant or metallic powder coatingcomposition.

Here, it is preferable that the supply temperature of the fluidizing airwhen the slurry 13 containing the binder is supplied be 80° C. or less.Moreover, it is preferable that the interior temperature of thefluidized bed when the slurry 13 is supplied be 50° C. or less. Underthese heating conditions, the resin powder is fluidized inside thetreatment chamber 1 for a predetermined time; then, the slurry 13 isintroduced, then the coating composition is dried, and then the productis cooled down to 40° C. or less. During this cooling, the bonding stateachieved under the heated conditions is fixed, and moreover the tuckingproperty at the surface of the resin powder, which has been increased byheating, is lowered to prevent flocculation or blocking of the coatingcomposition. Cooling may be performed outside the apparatus currentlydescribed. In a case where the product is air-transported immediatelyafter production, it does not necessarily have to be cooled immediatelyso long as it is kept fluidized.

In the drying process for removing the liquid (in this embodiment,water) that is supplied along with the coupler as the binder to thepowder coating composition, the product needs to be dried so that itswater content is 0.5% by weight, or further preferabley 0.3% by weight.This is because failure of appropriate removal of the liquid causes,during storage, problems such as fusion and blocking. Here, the watercontent denotes the content of a volatile component, be it water or aorganic solvent.

The spray nozzles 4 used to add the slurry 13 may be of any type. It ispreferable, however, to use a type that sprays as fine liquid particlesas possible. Moreover, it is preferable to operate the spray nozzles 4in such a way that, when water is sprayed, the size of the sprayedliquid particles is 100 μm or less on a D₉₀ (90% diameter) basis. Thespray nozzles 4 may be located below, above, or at the side of thefluidized bed, or may be distributed among two or more of thoselocations.

As shown in FIG. 1, the powder treatment apparatus of this embodiment isprovided with a filter 6 for collecting particles when air is exhaustedout of the treatment chamber 1. This filter 6 is provided with abackwash mechanism 7 for shaking off the particles attached to thefilter 6. In this embodiment, used as the filter 6 is a bag filter 6 a.The filter 6, however, does not necessarily have to be a bag filter 6 b,but may instead be, to name only a few, a common filter such as asintered metal mesh, or a cyclone, or a rotary rotor that performs airclassification. In FIG. 1, an arrow “a” indicates the filter backwashair supplied to the backwash mechanism 7. Above the treatment apparatusmain unit 2, there are provided: an air exhaust port 8 via which air isexhausted out of the treatment chamber 1; and an exhauster 9.

When air is exhausted out of the treatment chamber 1, particles arecollected by the bag filter 6 a provided in an upper part of thetreatment chamber 1. At predetermined time intervals, the backwashmechanism 7 sends pressurized air to the bag filter 6 a instantaneouslyin the reverse direction to perform backwashing so that the particlesattached to the bag filter 6 a are again fluidized to circulate insidethe treatment chamber 1. In this way, an instantaneous pressure isapplied to the bag filter 6 a from the air exhaust port 8 side thereofto the treatment chamber 1 side thereof to shake off the particlesattached to the bag filter 6 a.

As described above, in this embodiment, slurry 13 prepared by mixingtogether, by dispersion achieved by the use of a wet ball mill 19, apigment (or metal foil powder) in the form of fine powder or fine flakesand a binder is supplied in the form of spray to the surface of a resinpowder fluidized in a fluidized bed. Thus, an inherently highlyflocculative pigment (or metal foil powder) can be dispersed in theslurry 13, and in addition the binder acts to increase the adhesionbetween the resin powder and the colorant pigment (or metal foilpowder). Thus, it is possible to increase the dispersiveness of the finecolorant pigment or metal foil powder so that it surely coats thesurface of the base powder. In a powder coating composition produced bythe production process of this embodiment, the bonding strength betweenthe resin powder and the colorant pigment or metal foil powder is high,and thus even the powder retrieved for reuse contains a fixed content ofthe colorant pigment or metal foil powder. This makes the reuse of theretrieved powder possible.

Moreover, in the production of a powder coating composition with aparticular color (or metallic color), it is possible to minimize thequantity of pigment (or metal foil powder) to be added. Thus, withoutfurther adding the pigment or metal foil powder, it is possible toobtain an even color in the appearance of the finished coating.

Moreover, in this embodiment, it is possible to produce a desired powdercoating composition by separately preparing a resin powder as a base anda pigment (or metal foil powder) and then, to suit the desired quantityand color, coating the surface of the resin powder with slurry 13 bysupplying a spray of the slurry 13 to the resin powder fluidized in afluidized bed. Thus, there is no need to mix beforehand the resin powderas the base with the pigment or metal foil powder. This makes itpossible to add a color to a powder coating composition product at thelast stage of the production process thereof. Thus, when the color of acolorant or metallic powder coating composition is changed, it isnecessary only to clean the powder treatment apparatus. This helpsgreatly reduce the part of the production line that needs to be cleaned,and thus helps reduce the manpower and cost required for cleaning.Moreover, it is possible to produce colorant and metallic powder coatingcompositions suitable for wide-variety small-quantity production.

Furthermore, in this embodiment, the supply temperature of thefluidizing air when the slurry 13 having the pigment dispersed thereinis supplied is, preferably, 80° C. or less, and the interior temperatureof the fluidized bed when the slurry 13 is supplied is, preferably, 50°C. or less. Thus, it is possible to perform coating at the last stage ofthe production process of a powder coating composition product, when theoperation temperature is low. This helps prevent thermal deteriorationof the pigment, and also helps prevent the resin from gelating withincreasing temperature.

FIG. 2 is a vertical sectional view schematically showing, as a secondembodiment of the present invention, a powder treatment apparatus usedin a process for producing a colorant or metallic powder coatingcomposition. In the description of this embodiment, such parts as arefound also in the first embodiment described above are identified withcommon reference numerals, and no detailed explanation thereof will berepeated. In this embodiment, as in the first embodiment describedabove, air is blown upward into the treatment chamber 1 so that afluidized bed is formed in the treatment area A provided in a lower partof the interior of the treatment chamber 1; moreover, a liquid (a liquidin the form of suspended slurry) having dispersed therein a pigment (ora metal foil powder) in the form of highly flocculative fine powder orfine flakes is prepared, and a spray of this liquid in the form ofsuspended slurry is supplied into the fluidized bed so that the pigment(or metal foil powder) coats the surface of the powder material suppliedinto the treatment chamber 1. Here, fluidization is achieved by the useof fluidizing air and an agitator on a batch basis.

In this embodiment, as shown in FIG. 2, there are provided twomechanisms for spraying the slurry 13. Specifically, at the side of alower part of the treatment area A, a spray nozzle 4 is provided topoint inward, and, above the treatment area A, another spray nozzle 4 isprovided to point downward. By the action of pumps 14, the slurry 13,from two sources, is supplied to the spray nozzles 4, and simultaneouslyair is also supplied to the spray nozzles 4. This permits sprays of theslurry 13 to be supplied into the fluidized bed inside the treatmentarea A.

Furthermore, under the treatment area A, there is provided asubstantially disk-shaped agitator 15. The agitator 15 is rotated by theaction of an agitator motor 16. The rotation of this agitator 15cooperates with the fluidizing air to fluidize the resin powder. In thefigure, reference numeral 20 indicates a product takeout port formed inthe wall 2 b of the treatment apparatus main unit 2, and an arrow “a”indicates the filter backwash air supplied to the backwash mechanism 7.The conditions under which a coating composition is produced here arethe same as in the first embodiment. In this embodiment, with theconstruction described above, it is possible to obtain the same effectsas obtained in the first embodiment described previously.

Hereinafter, the present invention will be described in more detail byway of practical examples. It should be understood, however, that thepresent invention is not limited in any way to the examples specificallydescribed below.

PRACTICAL EXAMPLE 1

Used as the materials for slurry were: 50 g of a colorant pigment(“Pigment Yellow 83”, manufactured by Sanyo Color Works, Ltd.) with amean particle size of 14 μm; and 60 g of a binder liquid (“Superflex”manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) having a urethaneelastomer dispersed in water. These were, along with 890 g of water, fedinto a wet ball mill (“Aquamizer AQ-5” manufactured by HosokawamicronCorporation) to produce slurry. Used as the balls for filling theinterior of the wet ball mill were zirconia ball with an inner diameterof 3 mm. The wet ball mill was operated at a rotation rate of 250 rpmfor two hours. Through this treatment, the colorant pigment came to havea mean particle size of 2 μm or less, and slurry was obtained that hasthe pigment well dispersed therein.

On the other hand, used as a resin powder as a base powder was 1 000 gof a white polyester powder (manufactured by Kuboko Paint Co., Ltd.).This was fed into a fluidized bed coater (“Agglomaster AGM-2SD”manufactured by Hosokawamicron Corporation) so as to be fluidized with acurrent of hot air at 60° C. As the temperature of the resin powderrose, a spray of the slurry prepared as described above was suppliedinto the bed of the fluidized powder by the use of a fluid-type bottomspray nozzle provided in the fluidized bed coater. The slurry wassupplied at the rate of 15 g/min. As time passed, the resin powderbecame colored until, at the time that the entire quantity of the slurryprepared as described above was supplied in the form of a spray, apowder coating composition was produced that has the surface of theresin powder intensely colored in yellow. At this point, the supply ofthe spray was stopped. Consecutively, drying was performed with acurrent of hot air so that the moisture on the surface of the powdercoating composition was evaporated, then cooling was performed, and thenthe powder coating composition was collected.

Next, with the collected powder coating composition, coating wasperformed (test pieces of a mild steel sheet were coated therewith, andwere then baked at 180° C. for 20 minutes) by the use of anelectrostatic spray gun (manufactured by Hosokawa Wagner Corporation).Then, the appearance of the finish was inspected by sight.

COMPARATIVE EXAMPLE 1

Used were 50 g of a colorant pigment (“Pigment Yellow 83”, manufacturedby Sanyo Color Works, Ltd.) and 1 000 g of a white polyester powder(manufactured by Kuboko Paint Co., Ltd.). Under irradiation ofultraviolet rays, the colorant pigment and the white polyester powderwere mixed by agitation. Then, in an agitator-mixer, heated air wascirculated in the mixture powder so that the mixture powder wasfluidized, and thereby a powder coating composition was produced. Here,the irradiation of ultraviolet rays lasted 20 minutes. Next, with thecollected powder coating composition, coating was performed in the sameway as described above in connection with Practical Example 1, and theappearance of the finish was inspected by sight.

COMPARATIVE EXAMPLE

Used were 50 g of a colorant pigment (“Pigment Yellow 83”, manufacturedby Sanyo Color Works, Ltd.) and 1 000 g of a white polyester powder(manufactured by Kuboko Paint Co., Ltd.). These were mixed for 20minutes in a plastic bag, and thereby a powder coating composition wasproduced. Next, with the collected powder coating composition, coatingwas performed in the same way as described above in connection withPractical Example 1, and the appearance of the finish was inspected bysight.

The powder coating compositions of the practical and comparativeexamples presented above were evaluated in terms of coating appearance,coating workability, color/gloss, alkali resistance, and retrievability.The results are shown in Table 1. Alkali resistance was tested byapplying a 3 to 5% water solution of sodium hydroxide to the coatingsurface and then checking for color change. TABLE 1 Evaluation PracticalComparative Comparative Item Example 1 Example 1 Example 2 Coating Noflocculation Flocculation Strong Appearance observed in either observedflocculation base powder or in pigment observed pigment alone in pigmentCoating Neither pigment Both pigment Violent pigment Workabilityseparation nor spit separation separation and observed and spit frequentspit observed observed Color/Gloss Good Insufficient Unsatisfactorycolor opacity, intensity and uneven color Alkali Good Fair No goodResistance Retriev- Entire quantity Limited Unretrievable abilityretrievable retrievability due to due to pigment violent separationpigment separation

As shown in TABLE 1, Practical Example 1 yielded better results thanComparative Examples 1 and 2 in all items of evaluation. What wasparticularly notable with the powder coating composition of PracticalExample 1 was that neither the base powder or the pigment flocculated,and that the pigment evenly coated the surface of the base powder.Moreover, the coating exhibited good adhesion to the test pieces, andexhibited satisfactory surface strength. On the other hand, ComparativeExamples 1 and 2 suffered from flocculation of the pigment itself, andexhibited poor coating workability by causing pigment separation andproducing spit. In particular, Comparative Example 2 yieldedunsatisfactorily opacity and uneven color in the appearance of thefinish, and proved unretrievable due to violent pigment separation.

PRACTICAL EXAMPLE

Used as the materials for slurry were: 50 g of an aluminum foil powderwith a mean particle size of 20 μm; and 60 g of a binder liquid(“Superflex” manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) having aurethane elastomer dispersed in water. These were, along with 440 g ofwater, put in a beaker with a capacity of 1 L, and were agitated with astirrer to produce slurry. The operation duration was two hours.

On the other hand, used as a resin powder as a base powder was 1 000 gof a white polyester powder (manufactured by Kuboko Paint Co., Ltd.).This was fed into a fluidized bed coater (“Agglomaster AGM-2SD”manufactured by Hosokawamicron Corporation) so as to be fluidized with acurrent of hot air at 60° C. As the temperature of the resin powderrose, a spray of the slurry prepared as described above was suppliedinto the bed of the fluidized powder by the use of a fluid-type bottomspray nozzle provided in the fluidized bed coater. The slurry wassupplied at the rate of 15 g/min. As time passed, the resin powderbecame colored until, at the time that the entire quantity of the slurryprepared as described above was supplied in the form of a spray, apowder coating composition was produced that has the surface of theresin powder intensely colored in a metallic color. At this point, thesupply of the spray was stopped. Consecutively, drying was performedwith a current of hot air so that the moisture on the surface of thepowder coating composition was evaporated, then cooling was performed,and then the powder coating composition was collected.

Next, with the collected powder coating composition, coating wasperformed (test pieces of a mild steel sheet were coated therewith, andwere then baked at 180° C. for 20 minutes) by the use of anelectrostatic spray gun (manufactured by Hosokawa Wagner Corporation).Then, the appearance of the finish was inspected by sight.

COMPARATIVE EXAMPLE 3

Used as a resin powder as a base powder was 1 000 g of a white polyesterpowder (manufactured by Kuboko Paint Co., Ltd.), and used as a metalfoil powder was 50 g of an aluminum foil powder with a mean particlesize of 20 μm. These were fed into a fluidized bed coater (“AgglomasterAGM-2SD” manufactured by Hosokawamicron Corporation) so as to befluidized with a current of hot air at 60° C. On the other hand, alsoused was 60 g of a binder liquid (“Superflex” manufactured by Dai-ichiKogyo Seiyaku Co., Ltd.) having only a urethane elastomer dispersed inwater. As the temperature of the resin powder rose, a spray of thebinder liquid prepared as described above was supplied into the bed ofthe fluidized powder by the use of a fluid-type bottom spray nozzleprovided in the fluidized bed coater, and thereby a powder coatingcomposition was produced. Then, with the collected powder coatingcomposition, coating was performed in the same way as described above inconnection with Practical Example 2, and the appearance of the finishwas inspected by sight.

The powder coating compositions of the practical and comparativeexamples presented above were compared in terms of the same evaluationitems as with Practical Example 1 presented previously. The results areshown in Table 2. TABLE 2 Evaluation Practical Comparative Item Example2 Example 3 Coating No flocculation No flocculation Appearance observedin either observed in either base powder or base powder or pigment alonepigment alone Coating Neither pigment Neither pigment Workabilityseparation nor spit separation nor spit observed observed Color/GlossGood Slightly less gloss Alkali Good Good Resistance RetrievabilityEntire quantity Entire quantity retrievable retrievable

As shown in TABLE 2, Practical Example 2 yielded a better result thanComparative Example 3 in color/gloss. This is because, as a result of aspray of slurry having an aluminum foil powder evenly dispersed thereinbeing supplied to the surface of a resin powder as a base powder, thealuminum foil powder dispersed on the surface of the base powder, andpromoted bonding in their evenly aligned state, with the result that,after application, the aluminum foil powder exhibited even alignment.

The embodiments described above deal with either a process for producinga colorant powder coating composition that uses slurry having a pigmentin the form of fine powder or fine flakes dispersed therein or a processfor producing a metallic powder coating composition that uses slurryhaving a metal foil powder dispersed therein. It should be understood,however, that the present invention can be applied to a process forproducing any other type of powder coating composition, for example aso-called color-metallic powder coating composition that uses slurryhaving both a pigment in the form of fine powder or fine flakes and ametal foil powder mixedly dispersed therein.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a spray ofslurry prepared by mixing together a pigment and a binder by dispersingthe former in the latter is supplied to the surface of a resin powderfluidized in a fluidized bed. This makes it possible to disperse ahighly flocculative pigment in the slurry, and also, by the action ofthe binder, to increase the adhesion between the resin powder and thepigment. Thus, it is possible to increase the dispersiveness of afine-particle pigment so that it surely coats the surface of a basepowder. Moreover, it is possible to produce a powder coating compositionsuitable for wide-variety small-quantity production.

1. A process for producing a powder coating composition, wherein a resinpowder as a base powder is fluidized and a liquid in a form of slurryhaving a pigment dispersed therein is supplied in such a way that aspray of the pigment hits the fluidized resin powder so that the surfaceof the resin powder is coated with the pigment.
 2. The process of claim1, wherein the liquid in the form of slurry has a binder mixed therein.3. The process of claim 2, wherein the pigment is a pigment in a form offine powder or fine flakes, or a metal foil powder, or a mixturethereof.
 4. The process of claim 3, wherein the resin powder has a meanparticle size of 5 to 50 μm.
 5. The process of claim 4, wherein thepigment in the form of fine powder or fine flakes has a mean particlesize of 0.001 to 50 μm.
 6. The process of claim 5, wherein the resinpowder and the pigment in the form of fine powder or fine flakes aremixed together in a ratio of, on a weight percentage basis, 100 of theresin powder to 0.5 to 40 of the pigment.
 7. The process of claim 4,wherein the metal foil powder has a mean particle size of 1 to 50 μm. 8.The process of claim 7, wherein the resin powder and the metal foilpowder are mixed together in a ratio of, on a weight percentage basis,100 of the resin powder to 0.5 to 15 of the metal foil powder.
 9. Apowder coating composition produced by the process of claim 1.